THE LONGIPENNES. 141 



birds in which the plumage is very dense, such as the genera Diomedea and Halieus, and the 

 family of the Unguirostres, that the aftershaft is entirely wanting. In harmony with this is 

 its remarkable size in those birds which are without down-feathers, whether only between the 

 contour-feathers or also on the 'spaces, such as the Gattinae, Passerinae, and many Picarice. In 

 these groups also it is lost when dense down-feathers make their appearance, as in Alcedo. The 

 Pigeons alone, of which, however, the tracts are broad and very densely feathered, have neither 

 down-feathers nor aftershafts, whereas the Diurnal Birds of Prey, like most of the Waders, are 

 provided with both ; but their tracts are narrow, and the bands of which they are composed 

 consist often only of two, and at the utmost of five or six rows. 



The general outlines of the tracts in the Natatores agree much more in all their members 

 than is the case, for example, in the Wading Birds. The general rule is, that the inferior tract 

 does not possess a distinct and perfectly free outer branch, and the dorsal tract is not interrupted 

 by a true gap. The Longipennes form exceptions to both rules, and thus approach the Scolopacince. 

 Then Diomedea, Colymbus and Eudt/tes have a true gap in the dorsal tract, and Una, Alca, and 

 Mormon, a half-free outer branch reaching as far as the knee-covert. A trace of this is found in 

 the Unguirostres, and more distinctly in the Tubinares. In all these cases the ventral portion, 

 which is often independent only from the extremity of the sternum, is usually not only very broad, 

 but dilated towards the sides in the middle of the belly ; a structure which occurs only in Water 

 Birds, and has already been very distinctly recognised among the Scolopacince (seep. 134). The 

 relative numbers of the remiges and rectrices are more various in this group. The former here 

 attain their greatest number, namely fifty, in Diomedea; they are usually thirty, and rarely 

 decrease to twenty-five, as in the small species of Colymbus. Of the tail-feathers it may be 

 asserted in general, that they are very small, soft and numerous, but nevertheless often vary in 

 number. Thus among the Unguirostres there are usually twelve, fourteen, sixteen, eighteen, 

 twenty, or twenty-four rectrices but also fifteen, seventeen, nineteen, twenty-one, or twenty-three ; 

 in which cases there is one more on one side than on the other. I found the largest number 

 in Aptenodytes, namely thirty, but they are remarkably small. Twelve large, strong rectrices are 

 possessed without exception only by the Longipennes : the small Petrels appear to possess ten, as 

 the smallest number ; but even among these the majority have twelve, and Procellaria gigantea 

 has fourteen. Among the Unguirostres and Pygopodes nearly every species has its own charac- 

 teristic number ; but among the Steganopodes, in which again they become stronger, the usual 

 number is twelve, Phaeton having sixteen, some species of Haliem fourteen, and Pelecanus 

 twenty-two, twenty-three, or twenty-four. In those birds which are constantly swimming, such as 

 the Tubinares and Steganopodes, the oil-gland is very large, and frequently furnished with more than 

 two orifices; in the other families it is of smaller size, and has only two -apertures. Neither the 

 circlet of oil-feathers, nor the gland itself, is ever wanting in this division. 



1. LONGIPENNES. 



The pterylosis of this family approaches very closely to that of the Scolopacince, and can 

 hardly be distinguished therefrom with certainty by any character. It consists, on the head, of a 

 continuous plumage, in which I only find spaces about the eye and the orifice of the ear. From 



